This invention relates to polymeric membranes and more particularly to polymeric membranes which are used as separators in alkaline cells.
The principal requirements of a battery separator used in a conventional alkaline electrolyte battery are maximum chemical stability and a low electrical resistance for maximum diffusion of electrolyte in order to minimize the internal resistance of the over-all assembly. Other necessary properties include, sufficient mechanical strength to withstand handling during assembly of the battery, shape or dimensional stability when wet with electrolyte, and sufficient density to resist piercing of the separator and possible short circuit of the cell by metallic growths.
Separators for alkaline batteries have been formed of cellulosic or modified cellulosic materials. For example, U.S. Pat. No. 2,317,711 (Apr. 27, 1943) to Andre, discloses a secondary battery in which the positive active plate is formed of a substance such as silver oxide, silver peroxide, or combinations thereof, the negative plate is formed of zinc, and separation of the respective electrodes is achieved by wrapping the positive plate within a membrane of regenerated cellulose. Although alkaline electrolyte battery separators made of cellulosic or modified cellulosic materials are satisfactory initially, extended exposure to standard alkaline electrolytes breaks down these materials. As a result, the separator loses its initial insulating properties and is pierced readily by the growth or "tracing" of the metallic components, thereby severely limiting the life cycle of a secondary battery of this type.
Microporous polymeric films have also been used as battery separators; the manufacture of such films is disclosed in U.S. Pat. No. 2,542,527 (issued Feb. 20, 1951) and U.S. Pat. No. 2,676,929 (issued Apr. 27, 1954). In these patented processes, polymeric materials such as polyvinyl chloride or polyethylene are admixed with finely-ground starch particles and the mixture cast into sheets. Thereafter, by successive treatments and washing in both acid and alkaline media the starch particles are hydrolyzed and dissolved, leaving voids of approximately the same size as the original particles. Principal disadvantages of this procedure are the difficulty in controlling the size of pores in finished membrane and the difficulty in achieving pores of the order of 1000-2000 A. Pores of this size are desirable in alkaline silver-zinc and similar cells.